Interior french doors are no job for a neophyte Do It Yourself'er

Photo/Tim CarterThese interior French doors just need trim, paint and hardware to be complete. Thatâs the easy part; installing the doors properly isnât as easy as it looks.

Q. I’ve decided to install some interior French doors. Since I’m a rookie carpenter, I’m intrigued by some prehung French doors I saw at a lumber yard. What’s your opinion of these doors?

What else can you share that will help me install interior French doors like you might do it? Surely there are some secret tips that make the job go faster and easier.

A. Installing interior French doors, assuming you’re a real rookie carpenter, is going to sap you of every bit of skill and patience you have at this point in your DIY career. It’s hard enough to hang a single prehung door, much less a set of French doors. I’ve installed many prehung French doors with great success.

The reason French doors are harder to install than a single door is that you have additional tolerances you must satisfy to make the result look professional. The reveal around all the doors and in between the two doors needs to be consistent and as equal as possible. This is much harder to achieve than you might think.

If you decide to do the job yourself, the first step is to inspect the doors at the store. Look at the hinge side of the doors and see if the reveal or gap between the doors and the jambs is pretty close all the way around the door. You want a gap of just about ‰-inch everywhere. If the jamb is bowed or the gap is wildly different, look at other doors in stock. You want everything in your favor before you take the door home.

ROUGH OPENING

The next thing to get right is the rough opening in the wall where the French doors will fit. I prefer to have my rough openings at least ½-inch wider than the overall unit size of the door. The unit size is the overall height and width of the frame that the doors hang from. The same is true for the height. I like having the rough opening ½- inch higher than the door frame’s overall height. Remember: This height distance is measured from the top of the finished floor. It’s always better to have the doorjambs sit on top of a finished floor than to install the door and then try to butt up flooring to the doorjamb.

Since the actual doors of the prehung unit are undoubtedly perfectly square, this means that if the reveal or spacing in between the doors and the frame are equal, then the overall door frame is square. It’s imperative that the two doorjambs are installed plumb, parallel and in the same plane. If you can make the rough opening so the vertical rough studs are plumb, it will really help you.

Not only does the rough opening need to be square and plumb, it’s also very important that the opening is not a helix. This means that the opening has to be straight and plumb in both directions. If one leg of the rough opening is not plumb, then the two doors, when closed, will not be even at the bottom. Do you still want to do this install? It’s now just getting interesting.

CHECK LEVELS

You’ll also have an easy time if the floor under the door unit is level from one side of the opening to the other. If the floor is out of level, you’ll have to precisely cut down the doorjamb on the high side of the floor the amount it’s out of level from jamb to jamb. If you don’t do this, you’ll have to shim up the one jamb off the finished floor, creating an ugly gap. Your margin of error here is less than one-sixteenth inch.

Once you get the frame and the doorjambs into the opening, tack it in place with 10-penny finish nails. Be sure to use thin shims to ensure the doorjamb is plumb. You’ll have to open and close the doors numerous times as you nail, to constantly check the reveal and spacing between the doors and the doorjambs.

After you’re satisfied that the door is installed and the spacing is perfect, I recommend installing a hidden screw under the top hinge of each door. I prefer to use a 2.5-inch long screw that’s driven through the doorjamb into the rough opening framing lumber. This screw ensures that the doors stay in position for years. There’s tremendous tension on the top hinge, and the screw will anchor the doors to the rough framing.

FINISH NAILS

You’ll also need a nail set tool to drive the finish nails below the surface of the doorjamb. Be very careful as you hammer the nails that the head of the hammer never touches the surface of the wood. You don’t want any rookie beauty marks on the doorjamb.

Once your French doors are installed to your satisfaction and that of your significant other, you need to be sure they don’t warp. It’s imperative that you paint the top and the bottom of your wood doors with a minimum of two coats of paint. This paint significantly slows the absorption of water vapor into the long vertical stiles of the doors. If too much water gets into the door, it can warp. All of Tim’s past columns and videos are available at AsktheBuilder.com. Discover answers to thousands of questions. Subscribe to get FREE alerts each time a new video or column is uploaded to AsktheBuilder.com.